1. Field of the Invention
The present invention relates to a method and apparatus for automatically monitoring an optical signal-to-noise ratio (hereinafter, referred to as xe2x80x9cOSNRxe2x80x9d) by using a polarization-nulling method in a wavelength division multiplexed (WDM) optical transmission system, and more particularly, a method and apparatus for automatically monitoring an optical signal-to-noise ratio in which an arbitrarily polarized optical signal including an unpolarized ASE noise is inputted to a rotating quarter-wave plate and then to a rotating linear polarizer so that a maximum power and a minimum power of the signal outputted from the rotating linear polarizer can be detected, and the detected maximum power and minimum power is used for monitoring the optical signal-to-noise ratio.
2. Description of the Related Art
Currently, as a wavelength division multiplexed (WDM) optical transmission technology has been put to practical use, a transmission capacity of an optical transmission system has been increased sharply to over 1 Tb/s. In order to reliably operate and manage such a wavelength division multiplexed (WDM) optical communication network of a very high capacity, a monitoring of a transmission performance of the optical transmission system is necessary, and the transmission performance of such a wavelength division multiplexed (WDM) optical transmission system can be monitored easily by measuring an optical signal-to-noise ratio of each optical signal being transmitted. The optical signal-to-noise ratio is a ratio of a power of an optical signal to a power of a noise included in a bandwidth of the optical signal, which enables recognition of the transmission performance of the optical transmission system.
There have been proposed technologies for measuring an optical signal-to-noise ratio (OSNR) by linearly interpolating an ASE noise level at a wavelength of an optical signal from an ASE noise level at both sides of the wavelength of the optical signal in articles including [xe2x80x9cOptical signal quality monitor built into WDM linear repeaters using semiconductor arrayed waveguide rating filter monolithically integrated with eight photodiodesxe2x80x9d by published by H. Suzuki and N. Takachio, Electronics letter, Vol. 35, pp.836-837, 1999] and [xe2x80x9cOptical performance monitor built into EDFA repeaters for WDM networksxe2x80x9d by published by K. Asahi, M. Yamashita, T. Hosoi, K. Nakaya and C. Konoshi, which is presented at optical fiber communication conference, February,. 1998] as a conventional optical signal-to-noise ratio measuring method.
FIG. 1 is a graph illustrating a principle of measuring an optical signal-to-noise ratio (OSNR) using a linear interpolation method.
Referring to FIG. 1, the linear interpolation method is a method of linearly interpolating an ASE noise level at a wavelength of an optical signal from an ASE noise level at both sides of the wavelength of the optical signal such as an extended doted line, and the optical signal-to-noise ratio (OSNR) can be measured by using an interpolated ASE noise level. However, in the wavelength division multiplexed (WDM) optical transmission system in which respective optical signals can pass through different paths and a different number of erbium-doped fiber amplifiers (EDFAs) at any time, the ASE noise levels at the wavelengths of the respective optical signals may be different each other as shown in FIG. 2.
In this case, since the ASE noise level at a wavelength of an optical signal interpolated linearly from the ASE noise level at both sides of the wavelength of the optical signal differs from a practical ASE noise level at the wavelength of the optical signal, there is a problem in that the optical signal-to-noise ratio cannot be measured precisely by the linear interpolation method.
Therefore, the present invention has been made in view of the above-mentioned problems, and it is an object of the present invention to provide a method and apparatus for automatically monitoring an optical signal-to-noise ratio (OSNR) in which an optical signal and an ASE noise included therein are separated from each other, and an power of each of the optical signal and the ASE noise is measured by using a polarization-nulling method which linearly polarizes an arbitrarily polarized optical signal in a wavelength division multiplexed (WDM) optical transmission system.
According to one aspect of the present invention, there is provided a method for monitoring an optical signal-to-noise ratio (OSNR) using a polarization-nulling method, comprising the steps of:
(a) linearly polarizing an arbitrarily polarized optical signal including an unpolarized ASE noise;
(b) separating the optical signal and the ASE noise from the linearly polarized optical signal including the unpolarized ASE noise to measure a power of the optical signal and a power of the ASE noise included in a bandwidth of an optical signal; and
(c) obtaining the optical signal-to-noise ratio (OSNR) using the measured optical signal power and ASE noise power.
Preferably, it may be characterized that the step (a) is performed by allowing the arbitrarily polarized optical signal including the unpolarized ASE noise to pass through a rotating quarter-wave plate.
More preferably, it may be characterized that the step (b) further comprises the steps of:
allowing the linearly polarized optical signal including the unpolarized ASE noise to pass through a rotating linear polarizer;
measuring a minimum power and a maximum power of the signal outputted from the rotating linear polarizer; and
measuring the power of the optical signal and the power of the ASE noise included in the bandwidth of the optical signal from the measured minimum power and maximum power of the signal outputted from the rotating linear polarizer.
According to another aspect of the present invention, there is also provided an apparatus for monitoring an optical signal-to-noise ratio (OSNR) using a polarization-nulling method, comprising:
a rotating quarter-wave plate adapted to linearly polarize an arbitrarily polarized optical signal including an unpolarized ASE noise more than four times during the 360 degree rotation of the quarter-wave plate to output the linearly polarized optical signal;
a rotating linear polarizer adapted to output an signal having a power varying with an angle between the polarization state of the linear polarizer and the polarization state of the optical signal including the unpolarized ASE noise outputted from the quarter-wave plate;
a measuring means adapted to a minimum power and a maximum power of the signal outputted from the rotating linear polarizer; and
a calculating means adapted to measure a power of the optical signal and a power of the ASE noise included in a bandwidth of an optical signal according to the measured minimum power and maximum power of the signal inputted thereto from the measuring means to obtain the optical signal-to-noise ratio (OSNR).
Preferably, it may be characterized that the measuring means comprises a photodetector adapted to convert the signal inputted thereto from the rotating linear polarizer into an electric signal to output the converted optical signal, and the calculating means comprises a computer or a microprocessor adapted to obtain the optical signal-to-noise ratio (OSNR) according to the electric signal inputted thereto from the measuring means.